1. Field of the Invention
The present invention relates to a gas sensor having a porous member through which gas is permeable, such as an oxygen sensor for measuring the oxygen concentration of the exhaust gases emitted from automobiles.
2. Description of the Related Art
With a view to reducing CO, NOx, HC contained in the exhaust gases emitted from automobiles, it has heretofore been practiced to dispose an oxygen sensor at an exhaust system and control an air/fuel ratio of an air/fuel mixture to be supplied to an engine on the basis of an output of the oxygen sensor.
An example of an oxygen sensor used for such air-fuel ratio control is a wide-range air/fuel ratio sensor which includes a measurement gas chamber between an oxygen concentration cell and an oxygen pump cell and a diffusion control layer between the measurement gas chamber and a surrounding measurement gas. The diffusion control layer is a porous layer through which gas penetrates or permeates and controls the diffusion of detection gas (exhaust gas) introduced from the outside into the measurement gas chamber.
In general, some fuels and engine oils for internal combustion engines such as automotive engines contain phosphorus (P), and if such fuels and engine oils containing phosphorus (P) are used, the exhaust gases having gaseous phosphorus particles are emitted from the engines. This, however, causes such a problem that the phosphorus particles (spattered or suspended component) adhere to the diffusion control layer to clog or stop up the pores through which gas penetrates.
Since the temperature of the exhaust gases to which the oxygen sensor is applied is usually considerably high so that there may occurs such a case in which when the phosphorus particles adhere to the diffusion control layer in use of the oxygen sensor the phosphorus (P) and the material constituting the diffusion control layer react to produce a substance which is in a liquid phase at the temperature in use of the oxygen sensor. By this, a glasslike substance which is a reactant of phosphorus (P) piles up on the surface of the diffusion control layer, etc. to stop up or clog the pores through which gas penetrates. Such "stopping up" or "clogging" causes a variation of the resistance to diffusion of gas, so it is no more possible to carry out accurate detection of the air/fuel ratio.
Against such a "clogging" problem, a counterplan of adjusting the porosity and the diameter of the pores of the porous material has been made but not entirely satisfactorily.
Particularly, as the regulations on the exhaust gas emission have been becoming more stringent in recent years, more accurate air/fuel ratio control is necessitated. Furthermore, the reliability over an elongated period of usage is also demanded, so much more improvement in the oxygen sensor is eagerly demanded.